The present invention relates to a method of forming an electronically conductive composite layer in a device containing solid electrolyte, preferably the device is a fuel cell.
High temperature electrical devices are taught in U.S. Pat. No. 4,490,444 (Isenberg). In this type of device, typified by a combination of electrochemical cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, doped oxides of the perovskite family, such as lanthanum manganite. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material The interconnection material may be made of a doped lanthanum chromite film. The generally used dopant is Mg, although Ca and Sr have also been suggested.
Both the electrolyte and interconnect material are applied on top of the air electrode by vapor deposition process, at temperatures of up to 1450.degree. C., with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, and vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection material, as taught in U.S. Pat. No. 4,609,562 (Isenberg et al.). A fuel electrode, which is applied on top of the electrolyte is also bonded to the electrolyte by vapor deposition; here, nickel particles are anchored to the electrolyte surface by the vapor deposited skeleton of electrolyte material, as taught in U.S. Pat. Nos. 4,582,766, (Isenberg et al.) and 4,597,170 (Isenberg).
U.S. Pat. No. 4,631,238 (Ruka), in an attempt to solve potential interconnection thermal expansion mismatch problems between the interconnect, electrolyte, electrode, and support materials, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example LaCr.sub.0.93 Mg.sub.0.03 Co.sub.0.04 O.sub.3, as a vapor deposited interconnection mat using chloride vapors of lanthanum, chromium, magnesium, and cobalt.
It has been found, however, that there are certain thermodynamic limitations in doping the interconnection from a vapor phase by a vapor deposition process between 900.degree. C. and 1400.degree. C. Also, the vapor pressures of the calcium chloride, strontium chloride, cobalt chloride, and barium chloride are low at vapor deposition temperatures, and the transport to the reaction zone can be a problem. Thus, magnesium is the primary dopant used for the interconnection material. However, magnesium doped lanthanum chromite, for example La.sub.0.97 Mg.sub.0.03 CrO.sub.3, has a 12% to 14% thermal expansion mismatch with the air electrode and electrolyte material.
U.S. Pat. No. 4,861,345 (Bowker et al.), in a completely different approach, taught depositing particles of LaCrO.sub.3, doped with Sr, Mg, Ca, Ba or Co and coated with calcium oxide or chromium oxide, on an air electrode, and then sintering at 1,400.degree. C. Here, the metal of the surface deposit diffused into the LaCrO.sub.3 structure. This process completely eliminated vapor deposition steps and the skeletal support structure.
None of the proposed solutions solve all the problems of thermal expansion mismatch, and problems associated with doping calcium, strontium, cobalt, and barium by vapor deposition, or of providing method of depositing a uniform, leakproof conductive layer on a variety of substrates in a simple and economical fashion. It is an object of this invention to solve such problems.